Method of underground burning



June 23, i964 M. A. WEISS ETAL 3,138,203

METHOD OF UNDERGROUND BURNING Filed March 6, 1961 Malcolm A. Weiss Edward S. Cohen George G. Binder, Jr. INVENTORS BY fi 64 7 ATTO NEY United States Patent 3,138,203 METHOD OF UNDERGROUND BURBHNG Malcolm A. Weiss, Union, N.J., and Edward S. Cohen and George G. Binder, Jr Tulsa, Okla, assignors to Jersey Production Research Company, a corporation of Delaware Filed Mar. '6, 1961, Ser. No. 93,291

7 Claims. (Cl. 166-4) The present invention is broadly concerned with improved thermal methods for the recovery of petroleum from underground reservoirs. The invention is more particularly concerned with a unique technique for recovering petroleum from undepleted subterranean reservoirs by in-situ combustion of a portion of the carbonaceous materials present in such reservoirs. The invention especially relates to an improved in-situ combustion method of recovering oil wherein a burning front is caused to channel toward one ormore producing wells preferentially along the lower boundary of the stratum.

In the recovery of oil from undergroundreservoirs, there have been numerous advances in operating techniques so as to substantially increase the recovery of oil.

Even when using the most advanced of these techniques,

however, appreciable quantities of oil remain unrecovered from such reservoirs.

For reasons of economics, national defense, and conservation of resources, there continues to exist a great interest in the improvement of oil recovery methods. Techniques receiving particular investigation include waterflooding, gas repressuring, the use ofv solvents, and various thermal methods. Among the thermal methods, an especially attractive concept is that of in-situcombustion. In this method, a combustion front is established within a reservoir in the vicinity of one or more input wells. A combustion-supporting gas such as air, pure oxygen, oxygen-enriched air, or the like is injected within the reservoir through one or more input wells in quantities sufficient to burn a portion of the reservoir oil and to move the combustion front progressively through the reservoir toward the output wells.

As the combustion front advances in response to the injectionof the combustion-supporting gas, the heat liberated by the combustion mechanism causes oil and water to vaporize within the reservior. Cracking of the oil and formation of coke frequently occurs. A portion of the oil and/ or coke is consumed by the process.

'ice

wells.

While the existing'methods of in-situ combustion are all effective to a degree in recovering oil from a subter- As water and oil are vaporized and driven forward by the combustion front, they condense in cooler portions of the reservoir. This heat-transfer mechanism causes the oil in place to experience a reduction in viscosity, and its 7 displacement from the reservoir is thereby facilitated. Ultimately, a mixture of oil, water, and gas is withdrawn from the reservoir through the output wells. The oil is subsequently recovered at the 'earths surface.

Another interesting embodiment of the in-situ combustion methodis the so-called reverse combustion'technique. In this technique, a zone of combustion is established in the vicinity of ,one or more wells within a reservoir. Thesewells, which eventually are converted to output wells, function as input Wells until the combustion zone has been established. V

Once the combustion zone has been started, air or other combustion-supporting gas is injected 'into the reservoir through one or more of the regular input wells. Thainjected gas travels throughthe reservoir and passes through the combustion zonetoward the output wells. In passingthrough the combustion zone,'the gas serves the com.-

bined functions of sustainingthe' combustion process and into the unburned regions.

ranean reservoir, it is a disadvantage of all such methods that the combustion front tends to finger through highpermeability strata instead of advancing uniformly.- A particular shortcoming lies in the fact that the combustion front in any given oil-bearing stratum tends to channel along the top of the stratum and to break through into the output wells (or input wells in the case of reverse combustion) at a stage when only a small volume of the stratum has actually been contacted and depleted of oil. Once such breakthrough occurs, process efiiciency decreases markedly in that further oil recovery becomes increasingly dimcult to realize. The drive gas channels through the burned-out region and bypasses the unburned portions of the reservoir. Utilization of the oxygen in the gas from this point on depends primarily upon its ability to diffuse from the burned-out region the gas bypassing tends to occur along the; upper bound.- aries'of a stratum within a reservoir, a gaseous diffusion mechanism must be depended upon for further depletion of the reservoir. This becomesincreasingly critical with increasing thicknessof the stratum. In short, utilization of the oxygen in the drive gas falls ofi rapidly, and free oxygen appears in increasing quantities in the output wells.

The gas bypassing or over-burning mechanism also gives rise to several dificult operating problems. For example, breakthrough of the combustionfront into the output w lls causes high-temperature conditions to exist ,case of a reservoir characterized by a single, thick oilbearing stratum, the combustion front is caused to travel substantially along the lower boundary of the stratum.

Alternatively, in the case of a reservoircomposed of several distinct oil-bearing formations or strata, combustion fronts may be caused, to travel along thelower boundary of each such formation or stratum.

In order to propagate a combustion front in accordance with this invention along the lower boundary of'one' or more reservoir strata, a number of techniques may be employedalone or in combination. I For example, the oxygen-containing drive gas may be injected at maximum 1 rates compatible with reservoirvconditions and equipment capacity. by using such rates, and by injecting the gas selectively (as by packers) into thelowermost portionsfl of individual strata, deliberate channeling along the lower 'jj boundaries of the strata ispromoted. I The use of such injection rates, ,of course, generallyf requires increased gas injection pressures. I I V 7 Other means-"Le, means other than that of increasing the pres'sure and rate of injection gas/or selective pk ice rnent thereof-may be used for propagating a combustion front along the lower boundary of a stratum. Thus-,the input and/or output wells may be ,openeds ol'ely; atthe bottom of each stratum ofinterest, as byperforating', fracturing, or the like- In the case of fracturing, the fractures may be extended part or all of the way between input and output wells.

Since, as explained earlier,

any conventional. fracturing material.

Still other techniques of increasing the gas permeability of the lower portion of an oil-bearing stratum in preference to the upper portions thereof will readily suggest themselves to persons skilled in the art. Of primary and critical interest, insofar as this invention is concerned, is the creation of a zone of increased permeability along the lower boundary of an oil-bearing stratum so as to permit the subsequent and selective travel of oxygencontaining, combustion-supporting gas therethrough.

Once a combustion front traverses a reservoir in accordance with the method of this invention and approaches the output wells, the over-all operating conditions are changed so as to maintain but no longer promote or propagate the channels or zones of increased permeability. Conditions of high temperature and free oxygen at the output wells are also simultaneously avoided. Oil recovery and oxygen utilization are thereby maximized.

The changes required in the operating conditions for the second phase of the method of this invention include decreasing the rate of oxygen injection, as by decreasing the injection pressure and/ or oxygen content of the combustion-supporting gas. More particularly, the rate of oxygen injection should be decreased such that complete utilization of the oxygen is realized, and also such that temperatures no more than about 400 F and preferably no more than 300 F., prevail Within the output wells. Oxygen analyses of the gas produced from the output wells, and temperature logs of the output wells, will serve as satisfactory controls for observing these criteria.

By operating in the manner of this invention, the time required to exploit a typical producing formation can be reduced as much as tenfold or more. This reduction results from the fact that the invention deliberately exploits a marked difference between the upward and downward heat transfer rates directed from a combustion front with in a formation. The upward rate may be as much as fifty times greater than the downward rate, and will generally be about ten to fifty times as great. This difference appears to result primarily from the mechanism of vaporization and condensation of water and crude oil components. In effect, more oil is heated faster, and oil production is thereby greatly stimulated. Oil in a formation above the heated channels formed in accordance with this invention becomes heated and less viscous, whereupon it drains by gravity into the channels and is driven to the output wells. In addition, exploitation time ismarkedly decreased, which results in a corresponding decrease in heat losses to formations and strata which surround the formations or strata of interest.

The process of the present invention may be more readily understoodby reference to the drawing which helps to illustrate the best manner contemplated for carrying out the invention.

Referring to the drawing, oil-bearing strata l and 2 are shown positioned below the earths surface intermediate shale formations 3, 4, and 5. Cased input well 6 extends from the earths surface to strata 1 and 2. Cased output well 7 also extends from the earths surface to these strata. The input and output wells are perforated or otherwise opened to strata 1 and 2 at points 8, 9, 10, and 11. Following this, the strata are fractured along their lower boundaries by the .injection of fracturing fluid through one or more of the above perforations. The fracturing fluid may be the combustion-supporting gas to be used subsequently for exploiting the strata, but it may also be Propping agents are preferably used with the fracturing fluid.

Combustion is then started within strata 1 and 2 by injecting air or other oxygen-containing gas through input well 6 and openings 8 and 9 into the strata. Chemical or electrical igniters may be used to help initiate combustion, but their use often will not be necessary. Frequently,'the injection of an oxygen-containing gas will in itself be suflicient to start combustion of the oil in place.

Injection of the oxygen-containing gas is carried out at the maximum injection rate compatible with the injection equipment and the injectivitycharacteristics of the strata. Further, in the event the combustion fronts in the two strata progress at substantially different linear rates, injection of the combustion-supporting gas into the strata may be carried out separately. For example, separate injection strings may be used in well 6; and packers or the like may be used to separate the strata.

As the combustion fronts approach well 7, gas, vapors, and other fluids flowing within the strata cause the temperature within the well to rise substantially. Temperatures in excess of about 300 F. are indicative that the combustion fronts are at a point such that the injection rates of oxygen into the two strata must be reduced as described previously.

During the phase of this invention when the oxygen is injected at a reduced rate, the process is characterized by a very effective oil recovery mechanism. Thus, heat is transmitted directly from the burned-out portions of the strata into the over-lying oil-bearing portions. The oil, upon being heated, looses viscosity and flows more rapidly toward the output well 7 by gravity into the burned-out portions. Here, it is picked up or otherwise driven by the flowing vapors, gases, etc. to the output well. A very effective over-all heat-transfer mechanism is also set up wherein oil and water vapors migrate within the unburned oil-bearing portions and transfer heat to the cooler oil in place in these portions.

The process of the invention may, if desired, be cariied out intermittently rather than continuously. Intermittent operation is particularly contemplated for that phase of the process carried out after the burned-out channels or zones of increased permeability have been created. Thus, the injection of oxygen-containing gas into one or both strata may be interrupted periodically to permit a heat soaking mechanism to occur in which heat from the burned-out regions diffuses thoroughly into the oil-bearing regions. Rather than completely interrupt the injection of gas, it will generally be preferable to carry out injection at a reduced rate to insure that gas communication between the input and output wells is not entirely interrupted. The process is terminated when economic production is no longer possible.

What is claimed is:

1. In a method of recovering oil from an underground oil-bearing stratum wherein an oxygen-containing gas is injected into the stratum through an input well so as to propagate a combustion front toward a spaced output well and to drive oil toward said output well, the improvement which comprises limiting fluid communication between said input well and said stratum to the lower boundary portion of said stratum, forming a zone of increased permeability extending along the lower boundary of said stratum from said input well toward said output well, injecting said gas preferentially into said zone at a relatively high rate until the temperature within said output well reaches about 400 F., thereafter decreasing the rate of injection of said oxygen-containing gas, logging the temperature within said output well, and controlling the rate of oxygen injection in accordance with said temperature log so as to maintain said output well at a temperature not in excess of about 400 F.

2. In a method of recovering oil from an underground oil-bearing stratum penetrated by a gas input well and a spaced oil output well, and wherein a combustion front is propagated between said wells by the injection of an oxygen-containing gas through said input well, the imtemperature within said output well, regulating the quantity of oxygen injected within said stratum in accordance with said temperature log to maintain the temperature within the output well at a value not greater than about 400 F., and withdrawing oil from said output well.

3. A method as defined by claim 2 wherein the combustion front is directed preferentially along the bottom of said stratum by injecting said oxygen-containing gas selectively into the bottom of said stratum at a relatively high rate until the temperature within said output well reaches about 400 F., and thereafter decreasing said rate of injection.

4. A method of recovering oil from an underground oil-bearing stratum penetrated by an input well and a spaced output well, which comprises initiating a combustion zone within said stratum between said wells, limiting fluid communication between said input well and said stratum to the lower boundary portion of said stratum, limiting fluid communication between said output well and said stratum to the lower boundary portion of said stratum, injecting an oxygen-containing gas at a relatively high rate through said input well into said stratum to propagate said combustion zone along the lower boundary portion of said stratum between said input and output wells and to drive oil toward said output well, logging the temperature within said output well, controlling the rate of oxygen injection in accordance with said temperature log to keep the temperature within said output well below about 400 F., and withdrawing oil from said stratum through said output well.

5. A method as defined by claim 4 which includes the step of forming a zone of increased gas permeability within said stratum along the lower boundary thereof to cause said gas to traverse said stratum preferentially along the lower boundary thereof.

6. A method as defined by claim 5 wherein the zone of increased permeability extends from said input well toward said output well.

7. In a method of recovering oil from an underground oil-bearing stratum wherein an oxygen-containing gas is injected into the stratum through an input well so as to propagate a combustion front toward a spaced output well and to drive oil toward said output well, the improvement which comprises perforating said input well only at the lower boundary portion of said stratum, fracturing said formation along the lower boundary portion thereof from said input well toward said output well, injecting said gas through said input well and into said fracture at a relatively high rate until the temperature within said output well reaches about 400 F., thereafter decreasing the rate of injection of said oxygen-containing gas, logging the temperature within said output well, and controlling the rate of oxygen injection in accordance with said temperature log, so as to maintain said output well at a temperature not in excess of about 400 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,874,777 Tadema Feb. 24, 1959 2,931,437 Smith Apr. 5, 1960 2,958,380 Schild Nov. 1, 1960 3,004,596 Parker et al. Oct. 17, 1961 OTHER REFERENCES McNiel, I. S., In, Moss, 1. T.: Oil Recovery by In-Situ Combustion, The Petroleum Engineer, pages B-29- B-42, July 1958. 

2. IN A METHOD OF RECOVERING OIL FROM AN UNDERGROUND OIL-BEARING STRATUM PENETRATED BY A GAS INPUT WELL AND A SPACED OIL OUTPUT WELL, AND WHEREIN A COMBUSTION FRONT IS PROPAGATED BETWEEN SAID WELLS BY THE INJECTION OF AN OXYGEN-CONTAINING GAS THROUGH SAID INPUT WELL, THE IMPROVEMENT WHICH COMPRISES LIMITING FLUID COMMUNICATION BETWEEN SAID INPUT WELL AND SAID STRATUM TO THE LOWER BOUNDARY PORTION OF SAID STRATUM, INJECTING OXYGEN-CONTAINING GAS THROUGH SAID INPUT WELL SELECTIVELY INTO THE BOTTOM OF SAID STRATUM, THEREBY PROPAGATING SAID COMBUSTION FRONT PREFERENTIALLY ALONG THE BOTTOM OF SAID STRATUM BETWEEN SAID INPUT AND OUTPUT WELLS, LOGGING THE TEMPERATURE WITHIN SAID OUTPUT WELL, REGULATING THE QUANTITY OF OXYGEN INJECTED WITHIN SAID STRATUM IN ACCORDANCE WITH SAID TEMPERATURE LOG TO MAINTAIN THE TEMPERATURE WITHIN THE OUTPUT WELL AT A VALUE NOT GREATER THAN ABOUT 400*F., AND WITHDRAWING OIL FROM SAID OUTPUT WELL. 